Nanofibrillar cellulose refers to isolated cellulose microfibrils or microfibril bundles obtained from cellulosic raw material. Nanofibrillar cellulose (NFC), which is also known by other related names, such as nanocellulose and microfibrillar cellulose, is based on a natural polymer, cellulose that is a renewable resource and abundant in nature. Nanofibrillar cellulose has many potential uses for example based on its capability of forming viscous gel in water (hydrogel).
Nanofibrillar cellulose production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers. Because of the production technique at low pulp consistency, the concentration of nanofibrillar cellulose in prepared dispersions is typically low, usually around 1-5%. The prepared fibril cellulose material is a dilute viscoelastic hydrogel. The material itself is usable as such in many applications, but transport costs at such a low concentration from the production site to the user become high. Therefore, the nanofibrillar cellulose is preferably concentrated before the transport, and the user at the destination can dilute the product to a suitable use concentration.
The cellulose in the pulp fibers can also be converted to many chemical derivatives. The derivatization takes place mostly by chemical reactions of the hydroxyl groups in the β-D-glucopyranose units of the cellulose polymer. By chemical derivatization the properties of the cellulose can be altered in comparison to the original chemical form while retaining the polymeric structure.
If cellulose in fibers is derivatized in a suitable way, the fibers are easier to disintegrate to the level of fibrils, nanofibrillar cellulose, because of weakened bonds between the fibrils. For this purpose the cellulose can be anionized or cationized. For example catalytic oxidation of cellulose by heterocyclic nitroxyl compounds (such as “TEMPO”, i.e. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical) produces anionic cellulose where part of C-6 hydroxyl groups are oxidized to aldehydes and carboxylic acids. Another method to produce anionic cellulose is carboxymethylation of cellulose molecules. Cationic cellulose can be produced by adding quaternary ammonium groups to cellulose molecules. The chemical modification allows to consume less energy per mass of cellulose to reach a given disintegration level, and even a disperger-type disintegrator having a series of counterrotating rotors can be used, as disclosed by international application WO 2013072559.
When using cellulose modification to make starting material, pulp which contains cellulosic fibers in suspension, is subjected to chemical modification to reach a suitable degree of substitution, whereafter the fibers are disintegrated to fibrils with nanofibrillar cellulose as product.
Because large amounts of water are involved in the manufacture of nanofibrillar cellulose, most important factor in cutting down the transport costs is to reduce the water content of the product before shipping. Thus, nanofibrillar cellulose is supplied in the form of a concentrate to the destination. However, it has proved difficult to remove water from the hydrogel of nanofibrillar cellulose. Depending on the cellulose type and the end dry matter of the transportable product, there may be also difficulties to redisperse the product at the destination. Too harsh a drying treatment may for example affect the redispersability of the nanofibrillar cellulose so that the original viscosity values of the “fresh” nanofibrillar cellulose are not attained.